The invention herein relates to methods for producing fine grained metal compositions for use in semi-solid metal forming and for semi-solid forming of shaped articles.
Semi-solid metal forming, i.e., forming a metallic alloy at a temperature between its equilibrium liquidus and equilibrium solidus temperatures, is a hybrid metalworking process combining the elements of both casting and forging/extrusion. One of the key elements for the successful operation of a semi-solid forming process is the microstructure of metallic alloy being thus formed. Hereinafter, the term xe2x80x9cmetalxe2x80x9d is used to designate a metallic alloy with a major metallic constituent (base metal) along with various amounts of intentional additions (metallic and non-metallic) that modify the property of the base metal, as well as trace impurities that are deemed to not greatly deteriorate the performance of the alloy when used to fabricate articles thereof.
Conventionally solidified metals cannot be utilized in a semi-solid condition, since a structure of dendritic network forms upon solidification in such metals. Cracks and segregates will occur when a conventionally solidified metal is formed in partially liquid/solid state. Previous studies have shown that raw material of a semi-solid forming process must have a structure comprised of globular or spheroidal grains contained in a lower melting alloy matrix. When heated to a semi-solid temperature, the globular solid phase is retained, suspended in the lower melting alloy liquid matrix.
Producing semi-solid raw material requires specialized techniques. Thermal transformation processes are disclosed in U.S. Pat. Nos. 4,106,956, 5,009,844 and 5,571,346 where solidified metal having a fine dendritic microstructure is heated to and maintained at a superheated temperature above the solidus temperature of the metal, while keeping its body in a solid shape. After the dendritic networks are thermally transformed into globular solid particles, the metal is then formed in semi-solid conditions into an article.
Vigorous agitation processes are disclosed in U.S. Pat. Nos. 3,902,544, 3,948,650, 3,954,455, 4,310,352 (mechanical stirring) and 4,229,210 (inductive electromagnetic stirring) where during billet casting, a metal is agitated while it is in the semi-solid state and then cooled to solidify, forming the primary solid phase comprising discrete degenerate dendrites or nodules while preventing the formation of interconnected dendritic networks. Among the various agitation processes, the magnetohydro-dynamic (MHD) casting process has been commercially applied for producing a variety of fine-grain (mean grain effective diameter about 30 xcexcm) aluminum alloy bars (diameters varying from 38 to 152 mm) which satisfy the requirements of semi-solid forming. However the agitation processes have practical limitations for casting bars with diameters less than about one inch due to very low productivity.
U.S. Pat. No. 4,415,374 discloses a xe2x80x9cSIMAxe2x80x9d (strain induced, melt activated) process to make raw material for semi-solid forging. In the process, a solid metal composition is prepared by heating a conventionally solidified and homogenized ingot to a temperature in the hot deformation range of the metal, followed by hot extrusion or hot rolling plus additional cold working, resulting in an essentially directional grain structure. By heating the composition to a temperature above the solidus and below the liquidus, its directional grain structure transforms to a partially solid, partially liquid mixture comprising of uniform discrete spheriodal particles contained in a lower melting liquid matrix. The heated alloy is then formed and solidified while in a partially solid, partially liquid condition, the solidified article having a uniform, fine grained microstructure.
In comparison with MHD casting, the SIMA process described in U.S. Pat. No. 4,415,374 provides an effective method for producing small-diameter alloy bars (diameters less than 38 mm or 1.5 in.) employed in semi-solid forging. For large sizes, however, the economics of the process are not competitive with those of MHD casting for most metal alloys. Furthermore, the procedure of the SIMA process is very cumbersome, comprising the five discrete operations: conventional casting, image homogenization, heating, hot working and cold working. The nature of the process limits its application on a practical and economical scale, for not only large size but also small size semi-solid raw materials.
Therefore, it is an object of the present invention to provide a more superior process for producing a fine-grained solid metal composition suitable for semi-solid metal forming.
It is another object of the present invention to provide a more economical process for producing a fine-grained solid metal composition suitable for semi-solid metal forming. It is a further object of the present invention to provide a process and apparatus for preparing, delivering and semi-solid forming the above precursor material.
In one aspect of the present invention, there is provided a process of continuous casting and rolling followed by liquid quenching for producing a solid metal composition and structure suitable for semi-solid forming. The process includes providing and delivering a molten metal alloy to a mold of a continuous caster, solidifying the molten metal alloy at a specific rate, continuously rolling the solidified metal to a specific total area reduction by passing through 4 to 12 rolling stands, quenching the solidified and rolled metal, and taking up the product either in coil or as short lengths. The solidifying rate is preferred to be in a range of 10 to 150xc2x0 C./s to provide a fine dendritic microstructure in the solidified metal, with the dendritic grain size in the range of 20 to 150 xcexcm and the dendritic arm spacing in the range of 2 to 30 xcexcm. Hereinafter, grain size is measured by mean grain effective diameter. The total area reduction of the continuous rolling is larger than 90% (equivalent to Mises effective strain of 2.3) to provide a fine-grained deformation microstructure in the rolled metal having a grain size less than 20 xcexcm and a subgrain size less than 2 xcexcm. The quenching retains the deformed fine grain structure in the cast and rolled material. Liquid quenching is preferred to obtain the fine grain structure.
The continuously cast and rolled metal composition is heated to a temperature between the solidus and liquidus temperatures to obtain a microstructure which comprises discrete spheroidal particles suspended in a lower melting liquid matrix. The term xe2x80x9csemi-solidxe2x80x9d refers to a microstructure of spheroidal particles suspended in a lower melting liquid matrix, where solid loading is between 10 to 90%. The semi-solid precursor material is then xe2x80x9cformedxe2x80x9d by one of the many metal forming processes. This forming process utilizing the precursor material is characterized by high tool life and lower requirements for forming pressure. The articles thus formed have near-net shape and possess superior mechanical properties. The article formed by using the precursor material is characterized by a fine grained microstructure, with discreet spheroidal shaped particles suspended in a lower-melting matrix. The present invention can utilize any size of the precursor material, and the preferred range is bar stock of diameters less than 50 mm.
The throughput of the continuous casting and rolling process is higher than an other known process for making semi-solid precursor material. A typical continuous casting and hot rolling line can produce precursor material at a rate of 6 to 8 tonnes per hour. With modern computerized control systems this productivity can be further enhanced. It has been found that with typical single-wheel casting systems, increasing the cross-sectional area of the cost bar increases the casting throughput; however, as this cross-sectional area increases, segregation of the alloying elements becomes more pronounced.
The another aspect of the present invention provides a method which combines the continuous casting and rolling process for preparing semi-solid precursor material with a process of semi-solid forming of this precursor material into shaped articles. The apparatus employed in this method includes a caster for continuous casting, rolling stands, means for sawing or shearing the rolled metal into slugs of required length, means for heating the slugs to a temperature between the solidus and liquidus temperatures of the metal, means for delivering the rolled and heated slugs to the forming machine, and means for semi-solid die casting or semi-solid forging the slugs into shaped articles.
At first, in the combined process, a molten metal alloy is fed into a mold of the continuous caster and solidified at a solidification rate of preferably 10 to 150xc2x0 C./s to provide a fine dendritic microstructure in the solidified metal, with the dendritic grain size in the range of 20 to 150 xcexcm and the dendritic arm spacing in the range of 2 to 30 xcexcm. The solidified metal then undergoes a specific total area reduction by passing through 4 to 12 rolling stands. The total area reduction of the continuous rolling is larger than 90% (equivalent to Mises effective strain of 2.3) to provide a fine-grained deformation microstructure in the rolled metal having a grain size less than 20 xcexcm and a subgrain size less than 2 xcexcm. The rolled metal is then on-line sawed or sheared into slugs of required length. Optionally, the rolled metal can be quenched before or after sawing. The slugs are then delivered to the heating means to be heated at a rate of 0.5 to 20xc2x0 C./s to a temperature between the solidus and liquidus temperatures of the metal, and holding the heated at the temperature for 1 to 30 minutes. The heated slugs are then delivered to the forming machine. Semi-solid die casting or semi-solid forging can be used to shape the heated xe2x80x9csemi-solidxe2x80x9d slugs at injection speeds of 0.5 to 15 m/s. The slugs are typically shaped by utilizing a hydraulically powered ram to force into a die or mold (as in die-casting) or by closed die drop-forming (as in forging). Optionally, the semi-solid forging can be multi-stage process where the slug is preformed. The preformed slug can then be either solid forged or semi-solid forged into the article.